1. The Invention
This invention relates to insulated concentric tubing and more particularly to insulated concentric tubing as used in wells. The use of such tubing is usually intended to prevent the loss of heat between the interior of the inside conductor and the exterior of the outside conductor. The tubing has application in wells where hot produced fluids flow upward through the inner conductor and in wells where hot fluids are pumped downward through the inner conductor. In either case, there is a need to prevent the transfer of heat from the inner conductor through the tubing materials to the annulus around the outside of the outer conductor.
2. The Prior Art
In a copending application of Stanley O. Hutchison, Ser. No. 182,364, filed Aug. 29, 1980 for Insulated Concentric Tubing, there is disclosed an improved concentric insulated tubing string having an inner conductor, an insulating annulus surrounding the inner conductor, and an outer conductor. That application discloses a concentric conductor tubing string that has couplings between adjacent sections connected to the outer conductor and an improved coupling construction providing thermal insulation at the coupling.
Other prior art patents have suggested insulating schemes for accomplishing thermal insulation between the inner conductor and outer conductor of a concentric tubing string. See U.S. Pat. No. 3,525,399, Bayless, issued Aug. 25, 1970, U.S. Pat. No. 3,608,640, Willhite et al, issued Sept. 28, 1971, U.S. Pat. No. 3,680,631, Allen et al, issued Aug. 1, 1972 and U.S. Pat. No. 3,720,267, Allen et al, issued Mar. 13, 1973.
A common failure of many of the prior art insulation schemes has been the mechanical failure of the insulating material frequently because of the collapse of the insulating material within the annulus. The failure may be caused by rough handling during transport to an installation site or during actual installation in a well and, in some cases, by sealing failures during use when formation liquids seep into the insulation zone causing the insulation to become fluid saturated thus losing its isulation quality. These failures have demonstrated a need for an insulating material in the annulus of a concentric tubing string that can withstand the hostile handling associated with the well environment and a method for placing that insulation in the annulus that will substantially insure that seepage of formation fluids will not destroy the insulating quality of the insulation.